In SCD, ROS and reactive nitrogen species (RNS) are potentially generated by sRBCs, leukocytes, platelets, endothelial cells and plasma enzymes; moreover, several studies show that the production of these molecules is linked to different mechanisms such as increasing activity of nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase and endothelial xanthine oxidase (XO), HbS auto-oxidation, heme and iron release, asymmetric dimethylarginine (ADMA), decoupling of nitric oxide synthase (NOS) activity and reduced bioavailability of NO [33,34,35,36,37,38]. Here, XDH is linked to Schnyder corneal dystrophy.